Muscle stem cells, also called Satellite Cells (SC) function to maintain tissue homeostasis and regenerate skeletal muscle. In the adult, the SCs are maintained in a quiescent state both by intrinsic mechanisms and extrinsic signals from the niche. The quiescent nature of stem cells preserves the stem cell pool from premature exhaustion. One fundamental question is: how is the identity of the quiescent stem cells established and maintained during the life of an organism? And how the niche contributes to maintaining this identity? The notch signaling pathway is a major regulator of stem cell quiescence. In this proposal we aim to 1) identify the cell source and the specific Notch ligands that maintain SC quiescence and 2) how Notch signaling changes in the activated SCs. We will delete Mindbomb1, an enzyme that activates Notch ligands, specifically in the muscle fiber to determine the role of Notch ligands under homeostatic conditions and their impact on the ability of SCs to function under a regenerative response in vivo. We will also delete Dll4, a Notch ligand in the muscle fiber as preliminary data suggests that Dll4 expression increases in the muscle fiber during postnatal maturation which correlates with the appearance of quiescent SCs. Preliminary data also shows that activated SCs on an intact muscle fiber accumulate Dll4 in their cytoplasm; which is lost in the absence of muscle fibers. This suggests that muscle fiber is required for the increase in Dll4 protein in activated SC. We will assay the activated SC's on muscle fibers for Notch targets to see if there is a decrease in Notch signaling and will perform loss and gain-of-function for Dll4 in the activated SCs to see if Notch signaling is affected. The specific aims of this proposal are: 1) To determine the role of Notch ligands in the muscle fiber to maintain SC quiescence, 2) To determine if Dll4 is the critical mediator of Notch signaling that maintains SC quiescence and 3) To determine the functional role of Dll4 in activated satellite cell daughters. Successful completion of this project will provide molecular insights into the regulation of SC quiescence by the Notch ligands in the niche acting in a non-cell autonomous manner and how this transitions to cell-autonomous signaling in activated SCs.